For a number of years it has been common practice to use a sonar-type device for counting migrating salmon in streams. One such salmon counter is described in U.S. Pat. No. 3,416,127 to A. S. Menin et al and assigned to the assignee of the present application. The counter shown therein, while quite successful, is expensive to manufacture, largely because of the requirement for a large number of separate upward-looking transducers and several separate receivers. Because of the large number of transducers, this counter is capable of counting accurately in a stream with a very high density of migrating salmon. The transducers are mounted on a ladder-like array structure which is positioned on the stream bottom so that the salmon swim over it. Because the array structure is comparatively flexible, it tends to conform to the contour of the stream bottom sufficiently well that no fish are not counted because of their swimming under the array.
Because of the use of the upward looking transducers, the salmon counter described above requires water of a depth not less than about three and one-half feet. And while it counts salmon quite accurately in muddy water, it has no specific means for eliminating debris from the counts; thus it was initially calibrated at a location by adjusting a sensitivity level to correlate with a visual count.
The large numbers of transducers, receivers and associated equipment tend to adversely affect the reliability of this prior salmon counter as well as increasing its cost. There is a need for a counter which is easily deployed into smaller, shallower streams than are appropriate for the prior salmon counter and which is less expensive, more reliable and capable of operating in a stream having a substantial amount of debris without giving a large number of false counts.